Useless math that become useful

I’m writing an article on Lychrel numbers and some people pointed out that this is completely useless.

My idea is to amend my article with some theories that seemed useless when they are created but found use after some time.

I came with some ideas like the Turing machine but I think I’m not grasping the right examples.

Can someone point me some theories that seemed like the Lychrel numbers and then become ‘useful’?

Solutions Collecting From Web of "Useless math that become useful"

The Quaternions were considered useless for a long time.

Anyhow, the set of all unit quaternions is a double cover of $SO_3(\mathbb R)$. This allows us to represent any rotation matrix by a quaternion, which is used now in computer games (instead of using 9 parameters to parametrize a rotation matrix, we can use only 3 for the quaternions).

You can read more here.

Quote from G. H. Hardy1

The Theory of Numbers has always been regarded as one of the most obviously useless branches of Pure Mathematics.

Just 30 years after his death, the RSA algorithm was introduced which is deeply rooted in number theory and is now important part of sending encrypted information electronically, e.g., over the Internet.

The solutions of the closest-packing problem, of how to most densely pack non-overlapping congruent $n$-spheres in $\mathbb R^n,$ have, for some $n>3,$ been found to have applications to error-detecting and error-correcting codes in digital transmissions.

In the book Knots by Kaufman there is an example of an application of knot theory (the study of homeomorphic embeddings of $S^1$ into $\mathbb R^3$) to statistical mechanics.